Telephone repeater



April 9, 1935. B. F. LEWIS ET AL 1,997,222

TELEPHONE REPEATER Filed Sept. 1, 1933 5 Sheets-Sheet l Jwwmwmw INVENTORS BFZeu/zs BY JL.%I/7C ATTORNEY April 9, 1935 B. F. LEWIS ET AL TELEPHONE REPEATER Filed Sept. 1, 1933 3 Sheets-Sheet 2 w @NN IANVENTVORS A .B.FL8U/LS BY JLMer'rillJ ATTORN EY April 9, 1935. B. F. LEWIS ET AL TELEPHONE REPEATER Filed Sept. 1, 1933 5 Sheets-Sheet 3 LU RE INVENTORS a HFLeu/lls BY JLfllerriZlcl ATTORNEY Patented Apr. 9, 1935 UNITED STATES TELEPHONE REPEATER BcnjaminF. Lewis, Brooklyn, and Josiah Leverett Merrill, Jr., Jackson Heights, N. Y., assignors to American Telephone and Telegraph Company, a corporation of New York Application September 1, 1933, Serial No. 687,874 16'Claims. (01. 179 -170 This invention relates to telephone repeaters,

and more particularly to two-way telephone repeaters for use on irregular two-wire'lines.

It is an object of thisinvention-to obtain an 5 eifective repeater gain greater than would be permitted by the balance obtainable between the lines with which the repeater is to be used and their respective balancing networks.

A second object of the invention is to provide this gain with a minimum of distortion or other objectionable eifects on the transmitted. speech. A further object is to minimize the possibility of harmful eifects due to the presence of noise on the circuit.

In attaining these objects, the applicants make, in general, such modifications in an ordinary type of two-wire telephone repeater that there will always be sufficient attenuation in the singing path of circulating currents within the repeater to reduce such circulating currents, which are occasioned by impedance unbalances between the associated lines and their respective networks, to negligible proportions. This attenuation is under the control of voice-operated devices, which can redistribute the attenuation between the two sides of the repeater so that transmission through it in one direction or the It is well known that with the ordinary type of telephone repeater for use with two-wire lines, the gain that can be obtained is limited by the impedance balance between the lines to which the repeater is connected and the artificial lines or balancing networks which are provided, and

which are intended to simulate the impedance of the lines. If at any frequency the impedances of the lines and their respective networks are the repeater, the intensity of the current being determined by the degree of that impedance dis-' similarity or unbalance, and by the gain of the repeater amplifying elements at that frequency.

5 If the gain is not too great with respect to the impedance unbalance, any circulating currents which start to flow will be quicklyv dampedout;

and will produce no noticeable effects on the per formance of the repeater. 'If, howeven at any frequency the relation of unbalances and gains 1 tions, serious distortion of transmitted dissimilar, 'a circulating current will flow within' is such that a circulating current once started receives a net increment in passing once through the repeater, such a current will build up to a point limited only bythe'power capacityof the amplifying elements, and'a sustained oscillation will result. The repeater is then said to be in a singing condition, and is practically inoperative for transmission purposes. Even at gains.

somewhat lower than required to sustain suchan oscillation, circulating currents of certain frequencies. maybe started by incoming waves, and may persist for an appreciable fraction of a second before being damped out.' Under these condiwill result. 1

-To avoid these effects, it iscustomary'to con struct'lines, with which repeaters are to be used, with great care to obtain a high 'degreeof uniformity. The impedance characteristic of a line with uniformly distributed constants is such that it can be closely simulated, over awide frequency range, by a comparatively simple network. Even a loaded line, in which inductance is inserted at periodic intervals, is not difficult to simulate over a considerable frequency range, provided the loading coils are all of substantially the same inductance, and are spaced at approximately equal intervals. At the higher frequencies, especially in the; vicinity of the so-called cutoff frequency of a loaded line,accurate simulation becomes much more difficult, so that it is customary to reduce the gain of therepeater at these frequencies to avoid the possibility of singing.

Under some conditions it is impossible to obtain a satisfactory balance. necessary to use therepeater on a line which was not originally intended'for such use, and which was therefore not constructed with particular care. Another example would be a repeater at; a switching point, where connection must be made to anytwo of a considerable number of lines of For example, it may be" speech 7 different characteristics, and compromise networks. must be used which will provide only-a rough simulation for any'of the lines. Under' such conditions as these, our invention will permit a substantially greater efiectivegain than scription is to be read with reference to the ac 'companying drawings, which indicates schematicallythe circuits and apparatus involved. Likereference characters designate like parts in the several figures of the drawing. Figs.. l, 2 and 3 represent difierent embodiments of the invention, eachhaving certain advantages under particular conditions.

In each of the figures of the drawing, A and A designate the telephone lines to which the repeater is connected, and B, B the artificial lines or balancing networks which are intended to simulate approximately the impedance of the respective lines. C and C are balanced transformers or hybrid coils as customarily used in this type of repeater, and D, D indicate the amplifying elements of the repeater for the two directions of transmission. B, B, C, C, D and D, as interconnected by the heavy lines J, J" and K, K in Fig; 1, make up substantially the essential elements of a well known form of repeater.

To such a repeater we add amplifier-detectors E and E, responsive to telephonic currents, and arranged to insert attenuation in one sideor the other of the repeater. may be of any well known type; they are indicated in Fig. 1 as including the familiar negativegrid type of :vacuum-tube detector, and in Figs. 2

and 3 as using a copper. oxide or similar type of full-wave rectifier.

Returning to Fig. 1, the inputs of the amplifier-detectors E and'E are bridged across the conductors J and J, through the resistances H1 and H1, respectively. The rectified output of E passes through winding L of relay F in the direction to operate that relay, and through winding M of relay F in such a direction as to oppose the operation of that relay. Similarly, the rectified output of E passes through winding" L in the direction to operate relay F, and through winding M in the direction to oppose the operation of relay F.

.When no telephonic currents are reaching the repeater; singing is prevented by the substantially equal attenuation elements H2- and H2, one in each side of the repeater. H2 and H2 are shown connected. across conductors K, K through the transformers I and I, respectively; these trans formers are provided because the circuit is carry-' ing direct current. The circuit through H2 is closed through the back contact of relay G, and' similarly the circuit through H2. is closed through the back contact of relay G2. These relays are held against their back contacts by biasingcur rent'fiowing through windings R and R in series, as 'longpas current does not fiowthrough windings Q, P and Q, P, respectively, which windings are in the direction to operate the relays. Windings Q and Q are shoit-circuited by the back contacts, and windings P and P are opencirouited at the front contacts of relays F and F respectively. As long as relays F and F are normal, therefore, relaysG and G will also be normal, and losses Hz and H2 will both be connected to the circuit. Relay Fis held against its back contact when no telephonic currents are being received, by current from the plate battery in amplifier-detector E, flowing through winding Mand variable resistance N to ground. None of this current willflow through winding L under fier-detector E, through winding M and resistance N to ground.

'Iftelephonlc currents reach the repeater over These amplifier-detectors line A, a portion of them will enter E and be amplified and rectified. An increased current will then flow through winding M, increasing the bias against operation of relay F, and a portion of it will flow through winding L of relay F. If the incoming telephonic currents are sufiiciently intense, relay F will operate. This removes the short circuit from winding Q of relay G, and closes the circuit through winding P; both of these windings then carry current in the direction for operation, and relay G will operate. The breaking of the back contact of relay G removes loss H2 from the circuit. The closing of its front contact accomplishes two things; it connects resistance H1 across conductors J, and it short-circuits the input of amplifier-detector E. The value of H1" is so chosen as to introduce a loss in the circuit substantially equal to that which was removed by the disconnection of H2. Singing of the repeater is thus prevented by the losses H1 and H2, both in the lower side of the repeater, while the upper side of the repeater provides a, net gain for the telephonic currents being transmitted from line A to line A. The short-circuiting of E prevents any portion of the voice currents which might be transmitted through coil passing through to line A, singingwill be prevented by losses H1 and H2, and amplifier-detector E ,will be; disabled. As soon as the telephonic currents cease to arrive, relay F will release, since there is a steady biasing current flowing through winding M and resistance N. The circuit through winding P to ground will then be broken, and winding Q will be short-circuited. The current through winding P will not immediately fall to zero, however, since it will be sustained by inductance S and condenser T. As long as this current remains greater than that in the opposite direction through winding R, relay G will remain operated. Hence, by proper 'proportioning of inductance VS, condenser T and resistance U to each other and to winding P, the speed of release of relay G can be controlled, within limits. It is desirable that there should be a certain amount of delay in the release of relay G, since relay F may sometimes fall back between words, oreven between syllables, and the effective gain should not be allowed to change in this fashion. On the other hand, the release should not be made too slow, or complications may arise if telephonic currents reach the repeater from the opposite direction before the re-' lay releases. Thus, if the device is operated for transmissionv from A to A, and telephonic currents arrive from A while relay G is'still operated, they will be unable to affect amplifier-detector E because of the short circuit on its input. Such currents, will, however, be amplified through D, and a portion of them will pass through coil C into conductors J, by reason of the :unbalance between A and B. These currents can then affect amplifier-detector E, audit of sufficient intensity may hold relay F- operated, thus maintaining the losses H1 and H2" in their own path.'

If relays L and G release properly the'circuit is thenin a position to receive speech power from either direction; loss H1 will be removed, and loss Hz inserted. Currents arriving from line A will then operate the device in a manner exactly analogous to that already explained for currents ari'iving from A. Relays L? and G will operate, removing loss I-I2',.inserting loss H1, and shortcircuiting the input of amplifier-detector E. A net gain will thus be provided through the'repeater in one direction or theother, accordingto the direction from which speech currents are arriving,

while suflicient loss to prevent singing is. at'all times maintained in the singing path.

In cases where the false operation referred to above may be serious, or where steady operation in one direction may be anticipated dueto'steady noise on the lines, the circuit .indicated in Fig. 2 of the drawing may be preferable. In this circuit the two relays F and F arereplaced by'a single polarized relay designated as F. .The operating windings L and L of this relay are connected in the output circuits of amplifier-detectors E and E, respectively, and are so Wound that the fluxes due to currents in the two windings oppose each other. The arrangements for short-circuiting the input of one or the other of the amplifier-detectors are omitted in this system, so that both of them are at all times sensitive to incoming telephonic currents. The direction of operation of relay F will therefore be determined by the direction of the net flux due to the opposing windings L and L that is, by the winding which is carrying the stronger current. This in turn will dependupon the direction from which the more intense telephonic currents are reaching the repeater. Thus if the device is held in one direction by steady noise on the circuit, the arrival from the other.

direction of speech currents of intensity greater than that of the noisewill be sufficient to reverse relay F and give the gain of the repeaterin the,

, with this circuit the repeater is always set to give full gain in one direction or the other. Biasing windings V and V are provided to hold the armature firmly against the contactin the direction in which it is operated; each contact serves v to short-circuit the one of these windings which tends to pull the armature in the other direction. Thus, when the relay operates in one direction, it remains so operated until telephonic currents arrive from the other direction of sufilcient intensity to overcome the biasing flux. The sensitivity of the device may be adjusted by varying the biasing current by means of resistances W and W. V

This adjustment can also eliminate the'effects of diiierences in response of the two amplifier.- detector units at low values of voice-frequency input, which are just sufficient to operate the relay. Thus the sensitivity of the device can be made closely the same in the two directions. It is desirable to have a second adjustment for high values of input, since'the diiferences between. the amplifier-rectifiers may not be the same at high as at low inputs. Under these conditions, even with proper adjustment at low inputs,- false operation might be produced at high inputs. Such an independent adjustment is provided by means ,of the resistances X and X. In practice, the device is preferably tested first at'highinputs, and X and X are given such values as to equalize any differences between the two amplifier-detectors under these conditions. The input is then reduced to the value at which the device should just operate, and the'current in the biasing wind.

ingsV, V is adjusted by means of resistances W, W until the relay just operates on the desired value of input from either direction. This latter adjustment has virtually no. efiect on the performance of the system at high inputs. I

In the position shown in the drawing, the armature of relay F is in the position resulting from the arrival of telephonic currents over line A.

Winding Q of. relay G is short-circuitedrby the ground on the armature of relay F, and relay G is therefore held against its back contact by. the biasingcurrent through relay R, which opposes operation. Relay G, on the other hand, is held operated by current through winding Q, which overcomesthe biasing flux due to-the current in winding R. Loss H2 is therefore disconnected from'the circuit by the open back contact of rei Loss H2 is connected through relay. Thus, the repeater is in the position to amplify speech traveling from A towards A, and will remain in this position, whether or not tele phonic currentsare' arriving from line A, until currents arrive from A with sufficient strength to overcome the biasing flux due to the current in winding V, andcause the armature of relay F to move toits right hand contact.

When this occurs, the breaking'of the left hand contact will removethe short circuit from biasing winding V, andfrom'the operating winding Q of relay G. The current thus established through winding V will-assist in the movement of the armature to the rig'htjand will hold it firmly against the right hand contact when it arrives there. The current through winding Q .of relay G will overcome the biasing flux due to winding R, and operate the relay, removing loss H2 and inserting loss H1. Meantime, the arrival of the armature of relay F against its right hand contact short-circuits biasing winding V of relay F and operating winding Q of relay G. Relay G will therefore release, under the influence-of the steady biasing current through winding B. This will remove loss H1 and insert loss H2. The re peater is thus set'to amplify speech traveling from A towards A, and will remain in this position until the arrival of telephonic currents from A with suffi'cient intensity to reverse relay F. The entire process will then be reversed. I

-In maintaining a device-of this characten it is useful to be able to see at a glance whether or not it is operating approximately as it is intended to do. It is diflicult' to'see the movement of the relay armatures, since this movement is very slight I in relays of high sensitivity, and since they are ordinarily enclosed for protection against dust;

We propose therefore to provide lamps Y, Y,

connectedto the relay contacts as indicated, and so located as to be readily seen by the maintenance forces When the armature of relay F is againstits left-hand contact, as indicated in Fig. 2 of the drawing, lamp Y is short-.circuited through this contact, while: lamp Y is lighted.

Conversely,'when the armature is against its.

right-handcontact, lampY is short-circuited and lamp Y is lighted. During an ordinary conversation, therefore, the light will pass fromv one lamp to the other several times a minute. If,

however, one lamp burns continuously for several minutes, the probabilities are that the device is manently'in one direction, regardless of speech coming from the other direction. .Speechcure rents will almost always have frequent peaks, due to vowel soundswhich are much more intense than steady noise currents on any, commercial telephone circuit. These peaks will generally permit speech currents to take control of the device, even with considerable opposing noise, and to hold control of it, with the assistance of avery slight time lag in the operation of the relays, except perhaps during pauses between phrases. Furthermore, since the relays do not return to normal when notelephonic currents are flowing, there is no problem of adjusting time of release, as in thecircuit of Fig. 1, and no possibility that speech from one direction can hold the device operated against itself, as explained in the discussion of that circuit. 7 a 1" In one respect the arrangement'of Fig. 2 is def initely inferior to that of Fig. l. If the repeateris set, for example, for transmission from A toA', voice currents arriving-from A which are too weak to operate relay F will sufier attenuation due to both H1 and H2, instead of one loss only as in Fig. 1. Suchcurrents will frequently include those due to initial consonant sounds in a word; the first vowelsound will usually have sufficient power to operate the relay. To'minimize this effect, it will frequently be feasible to use attenuation elements which. cause a comparatively low. loss in the middle of the important voice-frequency range, and a'greater loss at higher and lower frequencies. This is indicated in Fig. 2 by showing theattenuation elements as made up of a combination of resistance, inductance and capacity. This arrangement is particularly suitable when the repeater is to be Used with loaded lines, since as mentioned in the foregoing general discussion, the unbalance between such a line and any simple form of network is usually much greater at the high frequencies, in the vicinity of the cutoff frequencies of'the line, than at lower frequencies. The most suitable attenuation eleents, therefore, would have a loss-frequency characteristic proportioned to the probable unbalance-frequency characteristic of the line or lines with which therepeater is to be used. Such attenuation elements could, of course,'be .used

with the circuit of Figfil as indicated by thenature of the pads H2 and H2 in that figure.

There is another. advantage of using attenuation elements havinga loss characteristic varying with frequency. As mentioned in the general discussion, it is customary to design telephone repeaters for two-wire'lines so that their gain decreases at the higher frequencies, becoming zerocutoff point is somewhat below that of the line.

These filters. necessarily introduce a certain amount Of'diStOltiQIlin the circuit. 'With the arrangement just explained, however, this loss combined with H1.

characteristic can be included in the attenuation elements, and the filter sections omitted from the repeater amplifiers, as indicated by the difference between amplifiers D, D, as shown in Fig. 1 and in Fig. 2. The distortion produced by these attenuation elements is, of course, confined to one side of the'repeater; and the side which is amplifying speech currents is substantially free from distortion. 1

With the arrangement of Fig. 2, since the losses on onev side of the repeater are either both in or both out at the same time, it is not essential that they be independently controlled, as in the circuit of Fig. 1. It will usually be desirable, however, to arrange that each ampli her-detector be connected at a point between the two loss elements, or,'if they are combined into a single element, at a point where part of the loss of that element is on each side of the amplifierdetector. The reason for this is as follows. Suppose that losses H2 and Hz were omitted, and the entire attenuation necessary to keep the repeater from singing concentrated in H1 and H1. The effect would be the same from the standpoint of transmission and singing, but false operation of the relays might result if one or both of the line'unbalances were large. Thus, with the relays in the position shown in Fig. 2, suppose voice currents arrive from line A. They receive the entire loss of the attenuation element before reaching the input of amplifier-detector E. Beyond that point they receive the gain of amplifier D, and r ach coil C at a considerably increased level, since loss Hz has been removed If the balance between line A and network B is low, there may be sumcie nt transmission through coil C to conductors J so that the level reaching the input of ampliher-detector E is equal to, or even greater than,

that at the input of E. Since transmission through the repeater is nearly instantaneous, there will be a current in winding I.- equal to or even greater than that in winding L, and relay F cannot reverse. Similar difficulties may exist if the loss elements are concentrated at any two symmetrical pointson the two sides of the circult;

Concentration of the losses into a single element for each side of the repeater will sometimes result in a desirable simplification in the relay system. Arrangements must, of course, be made to take care of the problem outlined in the preceding paragraph. Such a system is indicated in'Fig. 3 of the drawing. This is preferable to the systems of Figs. l and 2 for simplicity and speed or operation, but lacks certain features of adjustability and reliability.

In Fig. 3, the losses are shown as series rather than shunt elements, and the entire loss on each side is'inserted and removed by a single contact,

' so that the auxiliary relays G- and G of Figs. 1

and 2 can be omitted. Each loss is divided into two parts, H1, H2 and H1, H2, respectively, to

permit the connection of the amplifier-detectors at suitable intermediate points, as just explained. Control of the losses by a single relay is made possible by the use of a type of polarized relay in'which the steady pull of the permanent magnet on-the armature is sufficiently strong to hold the armature against the contact on either side to which it may be operated, without the assistance of auxiliary biasing current as in Fig. 2. It would not be feasible to connect the contacts of relay F in Fig. 2 directly to the loss elements,

resistance voltage characteristic.

W in Fig. 2.

as the battery for the biasingwindin'gs would cause clicks in the transmission circuit. 'I'he' E under sudden heavy peaksof current from the amplifier-detectors. In the circuit of Fig. 2 such chattering will usually have no effect on the] transmission, since relays G and G will not operateor release instantly. In the circuit of Fig. 3, however, the chattering'of the contact, is ap-' plied directly to the loss element. This effect can be minimized by applying to theoutputs of the amplifier-detectors E and 'E the pairs of copper-oxide or similar rectifier units Aa and Ac, each pair connected in parallel opposition through transformers, as indicated. Each pair constitutes a full-wave rectifier with a negative The more. intense the instantaneous peak of speech current, the greater-will be the shunting effect of the rectifiers, while the low-intensity speech. waves, which are just sufiicient to operate the. relay,

. will hardly be afiected at all. The tendency to in the condition established by telephonic currents arriving from line A. ,Losses H1 and'Hz are both short-circuited by the left-hand conktact'of relay F, and losses H1 and H2 are both in the circuit to prevent singing. Amplifier-detector E is connected across conductors 'J'Qat a point intermediate in the loss, the division of the loss being determined by the proportioning of H1 and H2 toeach other and to the impedances ofthe line and of amplifier D. Amplifier-detector E is efiectively; connected across conductors J through the impedance of H1 and H2 in parallel; by making the input impedance of E high in comparison with the impedances of H1 and Hz, the efiectot this onthe sensitivity of E can be made small. I Inc armature of relay F is held against the left-hand. contact by the pull of the permanent magnet, as already explained. H V,

On the'arrival of telephonic currents from line A, winding L of relay F will receive a larger current than winding L. If this differential is great enough to overcome the pull of-the permanent magnet, the armature will be swung'against the right-hand contact. Since the armature is now nearer to the right-hand pole-piece of the magnet than to the left-hand, the magnet will hold it in this position. The opening of the left-. hand contact will insert losses H1 and H2 in the upper side of the repeater, and the closing of the right-hand contact will ShOltrQiIClllt losses H1 and H2. The repeater will thus 'be set to amplify'currents transmitted from A towards A.

Transformers Z and Z may be iprovided'because the loss elements are inserted in one'side only of conductors J and J, respectively; ,This arrangement results in series unbalance of conductors J and J, and if such unbalanced 'cir-: cuits were connected'directlyto linesA andA',"

noisefmight be produced if longitudinal induc-.

tion were present on the line wiresr; Losses. H1 and H2 must be placed in series in the same con-1 ductor if they are to be controlled bya single relay contact; the same is true of losses 'Hii and H2. The sensitive, high-speed polarizedrelays which are adapted to the type of voice-operated system described in this application cannot ordinarilybe constructed with more'than'a single armature and twocontacts,

Whilethe invention has been disclosed in certain specific embodiments for the purposes of illustration', it will be understood that it niay be embodied other and'different forms without departing from the spirit of the invention, which is, defin'ediin the a pended claims. 4

What is claimed i'si v I 1. In' a 'two w'aysignaling system, the method or amplifyingthe signals at any point which consists in deriving at that point two separate unidirectional paths for transmission in the two directions, establishing a duplex balance against the line in eachdirection to reduce transmission from one, path into the. other, introducing the desired gain inone path, introducing inthe other path anattenuation such that, together with the effect of the duplex balances, it willprev'entobjectionable circulating currents through the two paths in' tandem without causing substantially more loss than is necessaryfor that purpose, determining the direction in which signals are being transmitted by an electrical comparison between the intensities. of the signal currents at corresponding points in the two paths, and interchanging thegain and loss between the, two

creasing the gain of the repeater-in the direction.

in which telephonic currents .are being transmitted and making a correspondingjdecrease'. in

the gain'in the opposite direction, the said direction of transmission being determinedby a continuous, electrical comparison between the intensities of the telephonic currents at-corresponding points in the two halves of the repeater.

3. In a two-way telephonerepeater having interconnected unidirectional paths for transmitting in the two. directions,. the method ofobtaining an effective gainwithout introducing objectionable circulating currents withinthe' repeater, which consists in introducmgthe desiredgain in one ofthe paths, introducing in the other path suflicient'loss at any frequencyv to prevent objectionable circulating currents within. the repeater at that frequency, deter- I mining-the direction of transmission of speech currentsby a continuous electrical comparison between the intensities of those currents at corresponding pointsin the two paths, andinterchangingthe gain and the loss between the two paths inaccordance with the direction of transmission so determined;

4. Ina telephone repeater for'use with twowire telephone lines, two separate unidirec-; tional paths for transmissionin the. two direc-- tions, an namplifying element in each'path, a

voice-operated device associated with each path and at all times responsive to telephone cur rents flowing therein,- a switching system adapt ed'to increase the gain of one or; the other of the. i

said amplifying, elements ,and to make a cor responding decrease in the gain of the other times respensiv to telephonic currents flowing.

vices as to make the operation of the said dif-' ferential means. dependent pnthe relative inten-' sities of the telephonic currents affecting the two voice-operated devices.

5. In a two-way telephone repeater for, use on' two-wire telephone lines, two separate. unidirectional paths for transmission in the two directions, an amplifying element with variable gm'n in each of the twopaths; a voice operated device-associat with eac Path and at all therein, meansflidiffer'entially connected to the two voice-operated devices. for determining which of saidfdeyices; is actuatedby telephonic currents of'the g reater intensity, and means controlled by saiddiiferentialmeans for making an increase in; the gain of orient the amplifying elements and a corresponding decrease in the gain of the other amplifying elements 6 In a two-way; telephone repeater 'foruselont twQ-wire telephone jline's, two separate i undi j rectional paths for transmissionin the'two di rections, an amplifying element and'anattenua tion element in each of the twopaths, 'a voiceoperat'ed device associatedwith reach amplifying element, a relaylcontrolled by'each voi'c'eroperated device,'a" switching system controlled by.

each relay adapted to remove the attenuation element from the associated path and to insert an additional attenuation'jelement in the other path, means for adjusting the sensitivity of the said relays, andelectrical means for adjusting the speed of operation switching systems. V

'7. In a two-way telephone repeater for connection to two wire telephone lines,'two sepa rate unidirectional paths for transmission in,

the two directions, an 'amplifyingelement in each pathg'an attenuation element associated with eachpath, eachattenuation element providing sufiicient loss at any frequency to prevent cbjectionable circulating currents through the repeater at that frequency, a voice-operated device associated 'with ea-ch path and atall times sensitive to telephone currents reach-.

' ing the repeater, and means differentially connected to the'two voice-'operated'devices and adapted to insert one or the other of the said attenuation elements in the path whose disociated with each path and including resistance, inductance and capacity, each attenuation element 'having a loss-frequency characteristic, when inserted in the associated path, soproportioned to the unbalance frequency characteris+ tics between the said lines and theirjrespective networks as to prevent objectionable circulating currents at any frequency without'causing a loss substantially greater than necessary for that purpose, and voice-operated means-for inand release of the said serting one or the other of-said attenuation elements in its associated path, said means being operated by'telephonic currents reaching the repeater from the said lines, its direction of operation being determined by the direction'of arrival' of the said telephonic currents at the repeater. t 9. In a telephone system, a two-way repeater connected, between two two-wire telephone lines and provided with two substantially distortionless amplifyingelements for amplification of telephonic currents intlietwo directions, voice-ope erated' means for determining the direction in which telephonic currents are being transmitted through the system, and means controlled by the said voice-operated meansfor reducing the gain of one or the other of said amplifying elements at each frequency. byan amount equal to, but not substantially exceeding, that necessary to prevent objectionable circulating currents through the repeater at that frequency.

"10. In a telephone repeater for use with twowire telephone lines, two separate unidirectional paths for'transmission in the two directions, an amplifying element in each path, a voice-operated device associated with each path and responsive to telephonic currents flowing in that path, a polarized relay difierentially connected to the, two voice-operated devices in such a way that its direction of operation is determined by the relative intensities of telephonic currents affecting the two voice-operated devices, means controlled by the direction of operation of the said relayffor inserting an attenuation element in one, or the other of said paths, and electrical means also controlled by'the direction of opera-. tion of. the said relay for holding that relay. in the direction last operated when no telephonic currents above a predetermined intensity are reaching therepeater from the lines.

11. In a telephone repeater for use with twowire telephone lines, two separate unidirectional paths for transmission in the two directions, an amplifying element in'each path, a voice-operated device associated with eaeh path and responsiveto telephonic currentsflowing therein, apolfar'ized relay differentially connected to they two voice operated devices in such away that its direction of operation is determined by'the relative intensities of their output currents, adjustable means for'proportioning the relative intensities of the output currents of the two voiceoperated devices tethe relative intensities of the telephonic currents flowing in the respective paths, and means controlled by the direction of operation of the said'relay for inserting an attenuation element in one or the other of the said paths, V

12. Ina telephone repeater for use with twowire telephonelines, two separate unidirectional paths for transmission in the two directions, an amplifying element in each path,'a voice-operated device associated, with each path and responsive to telephonic currents flowing therein, a polarized relay difierentially associated with the two voice-operated devices in such a way that its direction of operation is determined by the relative intensitiesof the telephonic currents flowing in the" two paths, means controlled by the direction of' operation of the said relay for inserting an attenuation element in one or the other of the 'saidpaths, biasing means for hold ing'th'e relay in the direction last operated, and

means for so adjusting the said biasing means that a predetermineddifierencebetween the teletion'element in each path, a voice-operated dephonic currents in the two paths will be required to operate the said relay.

13. In a telephone repeater for use with two- Wire telephone lines,two separate unidirectional paths for transmission in the two directions, an amplifying element in each path, a voice-operated device associated with each path and responsive to telephonic currents flowing therein, a polarized relay differentially associated with the two voiceoperated devices in such a way that its direction of operation is determined by the relative intensities of the telephonic currents flowing in the two paths, means controlled by the direction of operation of the said relay for inserting an attenuation element in one or the other of the said paths, and visual means for indicating the direction in which the'relayis operated.

14. In a telephone repeater, two separate unidirectional paths for transmission in the two directions, an amplifying element in each path, an

attenuation element associated with each path, a

voice-operated device associated with each path and at all times responsive to telephonic currents flowing in that path, and a polarized relay differentially connected to the two voice-operated vdevices in such a way that the direction of opvice associated with each path and responsive to telephonic currents flowing therein, a polarized I relay diflerentially connected to the two voiceoperated devices in such a way that its direction of operation is determined by the relative intensities of telephonic currents flowing in the two paths, the contacts of the relay being directly connected to the said attenuation elements in such a way as to remove one or the other of them from its associated path in accordance with the direction of operation of the relay, and magnetic means associated with the relay for holding the relay in the direction last. operated when no telephonic currents above a predetermined in tensity are flowing in the said paths.

16. In a telephone repeater, two separate unidirectional paths for transmission in the two directions, an amplifying element and an attenua-' tion element in each path, a voice-operated device associated with each path and responsive to telephonic currents flowing therein, a polarized relay differentially connected to the two voiceoperated devices in such a way that its direction of operation is determined by the relative intensities of telephonic currents flowing in the two paths, the contacts of the relay being directly connected to the said attenuation elements in such a manner as to remove one or the other of them from its associated path in accordance with the direction of operation of the relay, and means associated with each voice-operated device to limit the magnitude of the current flow:

ing in the windings of the said relay.

BENJAMIN F. LEWIS. JOSIAH L. MERRILL, JR. 

